Usually, polyolefin polymerization processes make use of a purge bin after the reactor vessel to remove unwanted volatiles from the polymer resin. The purge bin is a vessel where a resin mixture enters the upper portion of the vessel and is subjected to purging gas through ports or openings at the bottom of the vessel and possibly along the sides and other areas of the vessel to remove the volatiles through a purging effect. (See, for example, U.S. Pat. Nos. 3,797,707, 4,286,883, 4,758,654, and 5,462,351.)
However, simply piping a purging gas into the resin without taking into account solids flow distribution, distribution of the purging gas through the resin, resin flow patterns, and any potential heating effect of the purging gas can result in damage to the resin and possibly lead to poor or non-commercially viable polymer products. In addition, the time that the resin is exposed to the purging gas also affects the degree to which the volatiles are removed from the resin.
The interface between the resin and the purge bin along with any associated projections (such as pipes, tubes, supports, etc., that may protrude into the resin flow path) also has an effect on the flow rate of the resin through the purge bin. Since volatiles are purged from the resin at a rate dependent on the time of contact between the resin and the purge gas, any uneven flow pattern of the resin (i.e., slower or faster resin flow) will affect the amount of volatiles that are purged. Accordingly, the amount of volatiles removed differs from one portion of the purge bin to another dependent on the flow pattern of the resin.
Therefore, the resin flow through the purge bin should be decreased such that the portion of the resin that is exposed to the purging gas for the least amount of time (i.e., the portion of the resin moving the fastest through the purge bin), has sufficient contact time with the purge gas to remove the desired amount of volatiles. This decrease in the overall solids flow rate through the purge bin results in more time needed to purge the resin after polymerization, thus, decreasing the efficiency of the entire process.
Therefore, a more uniform cross-sectional flow rate of the resin through the purge bin is desirable to promote a more uniform resin residence time and dispersion of purging gas through the resin, so that less time is needed to purge the volatiles from the resin to an acceptable level.